In the field of wireless communications, in particular cellular communications, existing cellular telecommunications systems now support so-called 2.5G and 3G features. In this respect, support is now available for high-speed data communications in Global System for Mobile communications (GSM) networks, known as Enhanced Data rates for GSM Evolution (EDGE) or an Enhanced General Packet Radio Service (EGPRS).
To support EGPRS, wireless communications handsets, referred to as Mobile Stations (MSs) in some telecommunications standards, are typically equipped with a number of transmit modulator functions, typically two digital modulators. Consequently, it is known for a first modulator to support a Gaussian Minimum-Shift Keying (GMSK) modulation scheme and a second modulator to support an 3π/8 rotated 8 Phase Shift Keying (PSK) modulation scheme.
In order to support transmission of different types of information requiring use of different modulation schemes, for example data or logical channel signalling information, the wireless handset is capable of switching between the first and second modulators, for example on an adjacent slot of a multi-slot transmission. Such so-called multi-mode capability is known, for example from U.S. Pat. No. 6,834,084 and US patent publication no. 2002/0176514 A1.
U.S. Pat. No. 6,834,084 discloses a modulator capable of overcoming problems associated with use of linear power amplifiers, current demands of quadrature modulators and incompatibilities of transmit paths with certain existing standardised transmit methodologies. To this end, a polar modulator is disclosed comprising a polar converter, a digital predistortion filter and a phase locked loop. The polar modulator as described in U.S. Pat. No. 6,834,084 does not suffer from the previously known problem of non-contemporaneous arrival of phase and amplitude signals at the power amplifier.
In relation to the need to switch between modulators, the Third Generation Partnership Project (3GPP) Standard GSM 05.02 version 8.5.1 (ETSI EN 300 902 V8.5.1) provides for a guard band period, a period between information transmission bursts, to provide a time alignment margin for a base station receiver. During such guard band periods, 3GPP Standard GSM 05.05 version 8.5.1 (ETSI EN 300 910 V8.5.1) specifies that the response of a given transmitter adhering to the guard band must not contravene specified limits on peak power in spectral bands outside the RF band in which the given transmitter is operating. To achieve such attenuation of transmissions or when switching a modulator on or off, it can be necessary to cause the response of the given transmitter to ramp down to a condition of minimal power output from a condition of information-bearing modulation and then ramp back up to the condition of information-bearing modulation.
US patent publication no. 2002/0176514 A1 discloses a technique to control response of respective modulators used to provide EDGE, D-AMPS and GMSK modulation so as to control ramping up and down of the respective modulators. However, the profile of modulator response during ramping periods cannot be controlled easily, beyond the fixed ramping profile used.
Such inflexibility is disadvantageous, because the guard band period can, in some circumstances (such as an Access Burst followed by a Normal Burst having up to a 63 symbol timing advance in a multi-slot transmission), vary leading to shortened guard band periods during which the response of the modulator has to ramp up or down at a fixed rate, resulting in contention for processing time. Additionally, some power amplifier circuits used to amplify modulated signals do not ramp up and “settle” or ramp down and settle symmetrically in time.
In order to achieve the ramp profiles described in US patent publication no. 2002/0176514 A1, additional information, which is appended and pre-pended to information symbols, which include trailing bits, to achieve the desired ramping profile, needs to be transferred from the baseband processor to the modulator, resulting in a software burden on the baseband processor.
Furthermore, a number of known transmitters are not capable of strict compliance with a standardised slot size of 156.25 symbols/slot due to an inability of some modulators to re-start in sufficient time in the guard band period at a quarter symbol boundary of a given slot. Consequently, a symbol is inserted into a frame of slots every fourth slot, thereby maintaining a frame length as defined in the 3GPP Standard GSM 05.02 mentioned above. The need to insert an additional symbol every fourth slot creates a processing burden on the baseband processor to maintain the non-uniform slot lengths over a number of slots transmitted. Consequently, some guard bands are longer than others and there is a need to use a so-called “Layer 1 timer” in the baseband processor to monitor the guard band that is, effectively, extended in order to track a so-called power vs. time mask of the transmitter that has been adjusted as a result of the need to insert the additional symbol. However, when a need arises to switch between modulators, for example between a GMSK modulator and an 8 PSK modulator, the above-described solution, albeit problematic, is inapplicable, since a phase and amplitude discontinuities would occur between an undefined end point of phase and amplitude associated with the GMSK modulator and a starting phase and amplitude associated with the 8 PSK modulator.